5.4 URL ClassesThe URL classes provide the implementation and support of connections to resources using a Uniform Resource Locator or URL. A URL is a pointer to a resource on the network. The resource might be a file or a more complicated entity such as a database. A URL consists of a protocol type, a host name, a port number, a file name, and an optional anchor or reference. A typical URL might be http://www.madeupaddress.com:4567/index.html#page1 In this case the protocol is http, the host name is www.madeupaddress.com, the port number to which the TCP connection is made is 4567, the file name is "index.html" and the anchor is page1. If no port is specified, the default port for the specified protocol is used. The default HTTP port is 80, port 21 is reserved for FTP and port 23 is reserved for telnet. A relative URL contains a limited amount of information and relies on the information supplied by an absolute, or complete, URL to reach its destination. The URL class hierarchy is shown in the figure below.
5.4.1 URL classpublic final class URL extends Object implements Serializable Object URL InterfacesSerializable A URL object represents a Uniform Resource Locator or URL. URL() Constructor
The first version creates a URL object using a String representation of an absolute URL. The second version requires a URL object and a String representation of a URL. If the String does not represent an absolute URL, the URL object contextURL is used to fill in the missing pieces. The third version is the same as the second except a URLStreamHandler object is specified to be the stream handler for the URL object. The final three constructors create a URL object using separate arguments for the protocol, host, port number, filename, and stream handler. If the port and/or stream handler are not specified the default port and/or stream handler is used. equals() Method
equals() overrides the equals() method from the Object class and returns true if Object obj is a URL and contains the same URL as the invoking URL object. hashCode() Method
hashCode() overrides the hashCode() method from the Object class and returns a hash code for the invoking URL object. openConnection() Method
openConnection() returns a URLConnection object that represents a connection to the remote object referred to by the invoking URL object. openStream() Method
openStream() opens a connection to the URL contained by the invoking URL object and returns an InputStream object for reading from that connection. Methods to Return URL Properties
getAuthority() returns the authority portion of the invoking URL object. getContent() returns the content of the invoking URL object as an Object appropriate for the type of content. The second version allows the method to specify which classes it is looking for. getFile() returns the filename for the invoking URL object. getHost() returns the host for the invoking URL object. getPath() returns the path portion of the invoking URL object. getPort() returns the port for the invoking URL object or 1 if the port is not set. getProtocol() returns the protocol for the invoking URL object. getQuery() returns the query portion of the invoking URL object. getRef() returns the anchor for the invoking URL object. getUserInfo() returns the user info portion of the invoking URL object. sameFile() Method
sameFile() returns true if the invoking URL object has the same protocol, host, port, and filename as the URL object otherURL. toExternalForm() Method
toExternalForm() returns a String representation of the invoking URL object. toString() Method
toString() overrides the toString() method from the Object class and returns a String representation of the invoking URL object.
5.4.2 URLConnection Classpublic abstract class URLConnection extends Object Object URLConnection The URLConnection class is the parent of all classes that represent a connection between an application and a URL and provides methods common to those sub- classes. Sub-classes of the URLConnection class support-protocol specific connections to a URL. A URLConnection sub-class object can be used to read from and write to the reference represented by the URL. URLConnection is an abstract class; a connection object is created by invoking the openConnection() method of a URL object. connect() Method
connect() connects a URLConnection object to the resource specified by its URL object. getContent() Method
getContent() returns the content of the resource specified by the URL object that is associated with the invoking URLConnection object. The return value is an object that encapsulates the content of the connection. If a Class array is specified, the subclass of Object returned will be the first match of the contents of the Class array. getPermission() Method
getPermission() returns a Permission object representing the permission necessary to make the connection, or null if no permission is required to make the connection. getURL() Method
getURL() returns the URL object that is associated with the invoking URLConnection object. Request Property Methods
These methods allow properties associated with the URLConnection request to be queried or modified. The request properties are protocol specific. For information on the request properties for an HTTP connection, consult http://www.ietf.org/rfc/rfc2068.txt getRequestProperty() returns the value associated with the given key. setRequestProperty() specifies a key-value request property pair. Both the key and value are String objects. getDefaultRequestProperty() returns the default request property value corresponding to the specified key. This method was deprecated as of Java 2. setDefaultRequestProperty() sets the default key-value request property pair. This method was deprecated as of Java 2. Resource Property Methods
These methods return information about various properties of the resource specified by the URL object that is associated with the invoking URLConnection object by accessing the header fields of the resource. getContentEncoding() returns the content encoding of the resource. getContentLength() returns the content length of the resource, or 1 if it the length is not known. getContentType() returns the content type of the resource. getDate() and getExpiration() return the sending date and expiration date of the resource. The return value is the time in milliseconds since Jan 1, 1970, or 0 if the dates are not known. getHeaderField() returns the value of a header field. Either a header field index or header field name can be passed as an argument. getHeaderFieldDate() returns the value of a header field parsed as a time value. The return value is the time in milliseconds since Jan 1, 1970. If the header field cannot be parsed as a time, the specified defaultTime value is returned. getHeaderFieldInt() returns the value of a header field parsed as an int. If the header field cannot be parsed as an int, the specified defaultInt value is returned. getHeaderFieldKey() returns the name of the header field at index headerFieldIndex. getLastModified() returns the time in milliseconds since Jan 1, 1970 when the resource was last modified. guessContentTypeFromStream() attempts to identify the type of input stream by examining the first few bytes at the beginning of the stream. This method is useful because sometimes HTTP servers return an incorrect content type. Methods to Return Setup Parameters
getAllowUserInteraction() returns true if the connection associated with the invoking URLConnection object allows user interaction. getDefaultAllowUserInteraction() returns the default user interaction state. getDoInput() returns true if the invoking URLConnection object is to be used for input. getDoOutput() returns true if the invoking URLConnection object is to be used for output. getIfModifiedSince() returns the ifModifiedSince time in milliseconds since Jan 1, 1970. This time is used to determine whether a resource should be downloaded or if a current copy of the resource exists at the local host. getDefaultUseCaches() returns true if the use of caches is allowed by default. getUseCaches() returns true if the invoking URLConnection object uses caches. Methods to Change Setup Parameters
setAllowUserInteraction() specifies if the connection associated with the invoking URLConnection object allows user interaction. setDefaultAllowUserInteraction() sets the default user interaction mode of the invoking URLConnection object. setDoInput() is used to specify if the invoking URLConnection object will be used for input. setDoOutput() is used to specify if the invoking URLConnection object will be used for output. setIfModifiedSince() changes the ifModifiedSince time. This time, in milliseconds since Jan 1, 1970 is used to determine whether a resource should be downloaded or if a current copy of the resource exists at the local host. setDefaultUseCaches() specifies if the use of caches is allowed by default. setUseCaches() is used to specify if the invoking URLConnection object will use caches. Stream Methods
getInputStream() returns an InputStream object that can be used to read from the URL connection. getOutputStream() returns an OutputStream object that can be used to write to the URL connection. toString() Method
toString() returns a String representation of the invoking URLConnection object.
5.4.3 HttpURLConnection Classpublic abstract class HttpURLConnection extends URLConnection Object URLConnection HttpURLConnection An HttpURLConnection object represents a connection to a resource specified by an HTTP URL and provides support for HTTP-specific features. The HttpURLConnection class provides HTTP response code constants and methods for parsing server responses. HttpURLConnection is an abstract class; a connection object is created by invoking the openConnection() method on a URL object. HTTP Response Code Constants
These constants represent response codes sent by the server in response to a request. See the Sun Java documentation for more details. disconnect() Method
disconnect() closes the connection to the server. getErrorStream() Method
getErrorStream() returns an InputStream object representing the error stream if the connection failed but some useful information was sent by the server. The information could then be read from the InputStream. getResponseCode() Method
getResponseCode() causes the server to return one of the HTTP response codes, or 1 if none can be found. getResponseMessage() Method
getResponseMessage() returns the message returned by the server along with the response status. Redirects Methods
getFollowRedirects() returns true if the invoking HttpURLConnection object follows HTTP redirects. The default value is false. setFollowRedirects() is used to change the redirects mode. Request Method Methods
getRequestMethod() returns the current HTTP request method for the URL request. The default method is "GET". setRequestMethod() is used to change the method for the URL request. Valid arguments are "GET", "POST", "HEAD", "OPTIONS", "PUT", "DELETE", or "TRACE". usingProxy() Method
usingProxy() returns true if the URL connection is going through a proxy. 5.4.5 URLDecoder Classpublic class URLDecoder extends Object Object URLDecoder The URLDecoder class is used to convert from a MIME format called x-www-form-urlencoded format. to a standard ASCII String object. The x-www-form-urlencoded format is a subset of ASCII that can be read by a wide range of computer platforms. Its format is as follows: alphanumeric characters (a-z, A-Z, 0-9), the dash, the underscore, and the period are left unchanged. Spaces represented by the plus sign. All other characters are represented by the character '%' followed by a two digit hexadecimal number. decode() Method
decode() returns a standard ASCII String object that is converted from the x-www-form-urlencoded String passed as an argument.
5.4.6 URLEncoder Classpublic class URLEncoder extends Object Object URLEncoder The URLEncoder class is used to convert an ASCII String object into a MIME format called x-www-form-urlencoded format. The x-www-form-urlencoded format is a subset of ASCII that can be read by a wide range of computer platforms. Its format is as follows: alphanumeric characters (a-z, A-Z, 0-9), the dash, the underscore, and the period are left unchanged. Spaces represented by the plus sign. All other characters are represented by the character '%' followed by a two digit hexadecimal number. encode() Method
encode() returns a String object that is a x-www-form-urlencoded version of the String passed as an argument.
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